A universal law for all complex tasks: plan your novel, even if it is temporary. This action implies that you must take paper and pencil, and pass in writing, in the form of annotations, memories or more complex writings, the ideas you want to develop. You do not have to have the whole story in your head and pass it to the paper; simply, save the crucial points of the story you wish to write. If you can determine those central elements of your story, everything will be a little easier from then on. For example, if you have in mind a crime novel whose main element is human trafficking, focus on what you want to tell:

History or main plot: struggle of our protagonist against the mafias.

Secondary plot: a betrayal, a moral, a love affair with the woman of the mobster, a suicide, etc.

Scenarios: the port, police detachments, home of the protagonist, etc.

Personal life of the protagonist: divorced, married, widowed, boyfriend, lonely or assiduous to brothels, etc.

As it is logical, this is only a very elementary sketch so that you go getting into the subject and you manage to limit in your head the elements that will be developed. They can and must change a thousand times, but it is also good that you focus a bit on it and "work" on those planes. This point is linked, naturally, to the concept of documentation that we will explain next.